Since the discovery of stem cells, it has been understood that they have significant potential to effectively treat many diseases [1]. Pluripotent stem cells derived from embryos and fetal tissue have the potential to produce more than 200 different known cell types, and thus can potentially replace dying or damaged cells of any specific tissue [2, 3]. Stem cells differ from other types of cells in the body, and, regardless of their source, have three general properties: (a) they are capable of dividing and renewing themselves for long periods, (b) they are undifferentiated, and (c) they can give rise to specialized cell types.
Stem cells have been identified in most organs and tissues, and can be found in adult animals and humans. Committed adult stem cells (also referred as somatic stem cells) were identified long ago in bone marrow. In the past decade, committed adult stem cells have also been identified in tissues that were previously not thought to contain them, such as brain tissue, skin tissue, and skeletal muscle tissue [8, 9, 10, 11, 12, 13]. It was initially believed that adult stem cells are tissue-committed cells that can only differentiate into cells of the same tissue and thus regenerate the damaged tissue [1, 4, 5, 6, 7]. However, recent work suggests that adult organ-specific stem cells are capable of differentiating into cells of different tissues [8, 9, 10, 11, 14, 16]. Transplantation of cells derived from brain, muscle, skin and fat tissue has been shown to result in a detectable contribution in several lineages distinct from their tissue of origin [8, 9, 10, 11]. For example, recent reports support the view that cells derived from hematopoietic stem cells (HSCs) can differentiate into cells native to the adult brain [14], providing additional evidence for the plasticity of such stem cells.
The HSC is the best characterized stem cell. This cell, which originates in bone marrow, peripheral blood, cord blood, the fetal liver, and the yolk sac, generates blood cells and gives rise to multiple hematopoietic lineages. As early as 1998, researchers reported that pluripotent stem cells from bone marrow can, under certain conditions, develop into several cell types different from known hematopoietic cells [13, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27]. Such an ability to change lineage is referred to as cellular transdifferentiation or cell plasticity. Bone marrow-derived stem cells (BMSCs) have already been shown to have the ability to differentiate into adipocytes, chondrocytes, osteocytes, hepatocytes, endothelial cells, skeletal muscle cells, and neurons [28, 29, 30, 31, 32].
The process of stem cell differentiation is controlled by internal signals, which are activated by genes within the cell, and by external signals for cell differentiation that include chemicals secreted by other cells, physical contact with neighboring cells, and certain molecules in the microenvironment [33, 34]. For example, if embryonic stem cells are allowed to aggregate to form embryoid bodies, they begin to differentiate spontaneously. Embryonic cells of embryoid bodies can form muscle cells, nerve cells, and many other cell types [35, 36]. Although spontaneous differentiation is a good indication that a culture of embryonic stem cells is healthy, it is not an efficient way to produce cultures of specific cell types. In order to generate cultures of specific types of differentiated cells, e.g., myocytes, blood cells, or nerve cells, scientists must control the multiplication and the differentiation of stem cells by regulating the chemical composition of the culture medium, altering the surface of the culture dish, and/or by inserting specific genes.
Successful attempts have been made in vitro to induce differentiation of adult stem cells into other cells by co-culturing with other adult cells. For example, recent work has shown that co-culturing adult mouse BMSCs and embryonic heart tissue causes the BMSCs to both integrate into cardiac tissue and differentiate into cardiomyocytes (CMCs). Other work has shown that mesenchymal stem cells acquire characteristics of cells in the periodontal ligament when co-cultured with periodontal ligament tissue [37, 38].
Tissue injury may be one of the stimulants for the recruitment of stem cells to an injured site, by causing changes in the tissue environment, thereby drawing stem cells from peripheral blood, as well as triggering tissue replacement by locally resident stem cells. Some reports of elevated levels of chemokines and chemokine receptors such as CXCR4-SDF explain some of this in vivo stem cell recruitment [39]. Other reports suggest an important role of the chemokine CXCR8 (IL-8) as an anti-apoptotic agent which promotes tissue survival and induces recruitment of endogenous stem/progenitor cells [M, N, O]. An example of this mechanism can be seen in recent work showing that stem cells differentiate into liver cells when co-cultured with injured liver cells separated from the stem cells by a barrier [30].
CD31, the platelet endothelial cell adhesion molecule-1 (PECAM-1), is widely used as a marker during the development of endothelial cell progenitors, vasculogenesis and angiogenesis (A, B, C, D, E, F, H1). CD31 is constitutively expressed on the surface of adult and embryonic endothelial cells, is a major constituent of the endothelial cell intercellular junction (where up to 10^6 PECAM-1 molecules are concentrated) and is weakly expressed on many peripheral leukocytes and platelets (E, G, H). With a few minor exceptions, CD31 is not present on fibroblasts, epithelium, muscle, or other nonvascular cells. Independently of CD31 expression, endothelial cells and their progenitors are typically characterized by binding of Ulex-lectin in combination with the ability to uptake Acetylated-Low Density Lipoprotein (Ac-LDL) (I).
Regenerative medicine is an emerging scientific field with implications for both basic and practical research. Stem and progenitor cells are applied in a form of cellular therapy for local tissue repair and regeneration [41, 42]. These treatments aim to treat disorders in practically all tissues and organs, such as the bladder, intestine, kidney, trachea, eye, heart valves, and bones [43, 44]. Intensive studies are being conducted worldwide in order to generate stem cell-based tissue engineering therapies. These studies include experiments for the regeneration of blood vessels [13], bone [35, 45], cartilage, cornea, dentin, heart muscle [46], liver, pancreas [47], nervous tissue, skeletal muscle, and skin [18, 34, 48, 49]. Stem cell-based therapies can use cells from various organs in order to generate different tissues. For example, epithelial surfaces (taken from various tissues such as the skin, cornea and mucosal membrane) may be used as a source for corneal and skeletal tissues [50, 51]. Additionally, in a more widespread application, blood marrow-derived stem cells are used for regeneration of several different tissues such as bone, cartilage, adipocytes, neurons, and cells of the hematopoietic system [33, 42].
Stem cells can be administrated systemically or locally using injections to the injured site. However, other potential administration routes and usage of different medical devices are being developed and tested. Different medical devices such as chemical, metal or biodegradable based devices have been described for the administration of stem cells into the heart and blood vessels (J, K).
US Patent Application Publication 2004/0228847 to Goldschmidt-Clermont et al., which is incorporated herein by reference, describes stem/progenitor cells and, in particular, therapeutic strategies based on the use of such cells to effect vascular rejuvenation and/or to serve as delivery vehicles.
PCT Patent Publication WO 2005/120090 to Fulga et al., which is assigned to the assignee of the present patent application and is incorporated herein by reference, describes a method for use with extracted blood, including (a) applying blood to a first gradient suitable for selecting first-pass cells having a density less than 1.077 g/ml; (b) applying the first-pass cells to a second gradient suitable for selecting second-pass cells having a density between 1.055 and 1.074 g/ml; (c) increasing the number of cells having a density between 1.055 and 1.074 g/ml, by culturing the second-pass cells for a period lasting between 3 and 30 days; and (d) identifying endothelial progenitor cells in the cultured cells. Other embodiments are also described.
United States Patent Application Publication 2004-0228897 to Zhang et al., which is incorporated herein by reference, describes a medical device for use to assist stem cell and/or stem cell derivatives in repopulating, repairing and/or replacing the heart tissue in a failing heart muscle, in order to restore the heart's ability to pump blood. The medical device is made of biocompatible materials. The specific design of the device is described as facilitating the stem cells coated in the device to repopulate heart muscles inside the heart. Stem cells are attached to the coated device, proliferated and/or differentiated on the device in a bioreactor before implantation. The device also contains bioactive components that diminish rejection by the host's immune system. The device may be directly implanted into the failing heart muscle area to assist stem cells to repair failing heart muscles via surgical and/or percutaneous catheter based procedures. In another embodiment, the device may be implanted to the surgical site where abnormal heart muscles are removed, to assist stem cells to repopulate heart muscles, to replace the failing heart muscles.
US Patent Application Publication 2005/0209556 to Tresco et al., which is incorporated herein by reference, describes a device and method for the delivery of cells, tissues, enzymes and/or pharmacological agents for the treatment or prevention of diseases, disorders or deficiencies. The device is placed intravascularly and includes a chamber that houses living cells delimited by a membrane on either side that physically separates the cells from the blood stream and the central lumen of the catheter. The device can be inserted over a guidewire and permits flushing and reloading of the central lumen with viability supporting factors that sustain the cells in the outer chamber for long indwelling times without removing it from the body. In addition, the central lumen can be used to deliver therapeutic substances or withdraw blood. The new intravascular catheter is described as being able to be used for the treatment or prevention of a variety of diseases and disorders, and may use the implantation of living cells, tissues, enzymes or pharmacological agents. The device is described as being used, for example, for non-therapeutic purposes that may involve sustained intravascular release of biological factors as, for example, in stimulating growth of farm animals to augment the production of meat. Placement of cells within the device for release of angiogenesis, cytokines, enzymes, and other factors is described. The use of stem cells within the device is also described.
U.S. Pat. No. 6,810,286 to Donovan et al., which is incorporated herein by reference, describes a stimulatory device for the controlled production of angiogenic growth factors. More specifically, a subthreshold pulse generator is used for the local production of vascular endothelial growth factor.
The following references, which are incorporated herein by reference, may be of interest:
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